Generally, such a bumper beam comprises a curved cross-member, consisting of a structural or semi-structural element, for example a profile made of metal or composite material, and two side shock absorbers called “crash boxes”, such absorbers being shock absorption modules whose function is to absorb energy by deformation in case of shocks, especially during “Danner” or “Bumper test” insurance type shocks.
The shock absorbers are configured to be arranged between the cross-member and two longitudinal members of the vehicle. The absorbers are therefore attached to the cross-member by an assembly such as riveting, bonding, welding, or by spraying an adhesive at the interface, etc. The aim is to preserve the mechanical integrity of the connection between the beam and the absorbers during any type of shock.
One difficulty lies in the fact that the cross-member may in some cases be stressed in at least two different ways against shock absorbers. Firstly, it may be subjected to compressive stress on the side during a “Danner” type shock, during which the vehicle is for example projected at 15 km/h against a rigid wall inclined at 10° with 40% overlap. The aim is thus to obtain maximum absorption by the shock absorber and in this case the cross-member works in compression at the stressed absorber. The absorbers are also subjected to compressive stress. Note that, concerning the cross-member as a whole, a first mode of deformation of the cross-member in bending takes place before compression of the absorbers. Secondly, the cross-member may be subjected to bending stress, during a “pole” shock, or in its centre during a “Bumper test” type shock, also called new “IIHS” insurance shock during which the vehicle is projected at 10 km/h, being centered on a rigid shock impactor, similar to a barrier, consisting of a deformable element representing the rigidity of the shock system of a second vehicle in case of head-on impact. In this case, and in reaction to the centered impact, the shock absorbers are first subjected to tensile stress towards the front, especially when the beam is initially curved and loses its curve by deformation at the start of the shock, since the ends of the cross-member tend to detach from the side absorbers due to the reaction force. The absorber is in fact attached to the rear of the longitudinal member, generally by a plate, whereas the front of the absorber is pulled forward by the ends of the cross-member. This phenomenon also exists in case of a straight, not curved, cross-member. Since each of the absorbers is rigidly attached to the vehicle by means of plates, an impact centered on the cross-member induces a tensile reaction force at the absorbers. Secondly, the shock absorbers are subjected to compressive stress towards the rear, when the cross-member is no longer bent. Due to these various load cases, the connection between the cross-member and the absorbers is highly stressed, and the cross-member could become detached from the shock absorbers making the bumper beam less efficient and resulting in greater damage than that accepted by insurance companies.